Fuel cells are environmentally clean, quiet, and highly efficient devices for generating electricity and heat from hydrogen, natural gas, methanol, propane, and other hydrocarbon fuels. Fuel cells convert the energy of a fuel directly into energy—electricity and heat—by an electrochemical process, without combustion or moving parts. Advantages include high efficiency and very low release of polluting gases (e.g., NOX) into the atmosphere. Of the various types of fuel cells, the solid oxide fuel cell (SOFC) offers advantages of high efficiency, low materials cost, minimal maintenance, and direct utilization of various hydrocarbon fuels without extensive reforming. SOFC systems operating with natural gas as a fuel can achieve power generation efficiencies in the range of 40 to 45 percent, and even higher efficiencies are possible with hybrid systems. Power is generated in a solid oxide fuel cell by the transport of oxygen ions (from air) through a ceramic electrolyte membrane where hydrogen and carbon from natural gas are consumed to form water and carbon dioxide. The ceramic electrolyte membrane is sandwiched between electrodes where the power-generating electrochemical reactions occur. Oxygen molecules from air are converted to oxygen ions at the air electrode (cathode), and these oxygen ions react with hydrogen and carbon monoxide to form water and carbon dioxide at the fuel electrode (anode). Compositions commonly used for the ceramic electrolyte membrane material may include lanthanum-strontium-magnesium gallate (LSGM) yttrium-stabilized zirconia (YSZ), gadolinium-doped ceria (GDC), and samarium-doped ceria (SDC), among others.
The fuel electrode (anode) is a composite (cermet) mixture of a ceramic electrolyte material (e.g., YSZ, GDC, SDC, or a combination thereof) and a metal (e.g., nickel). The anode material typically is produced as a mixture of the electrolyte material (e.g., YSZ) and the oxide of the metal (e.g., nickel oxide); prior to operation of the SOFC, the nickel oxide in the composite anode is reduced to nickel metal.
The air electrode (cathode) is a ceramic material. Commonly used cathode compositions may include lanthanum strontium manganite (LSM), lanthanum strontium ferrite (LSF), lanthanum strontium cobalt ferrite (LSCF), samarium strontium cobaltite (SSC), praseodymium strontium manganite (PSM), and praseodymium strontium manganese iron oxide (PSMF), among others. The majority of the cathode materials considered for SOFC applications have a perovskite crystal structure. The perovskite system can be generally described as having the formula ABO3 where the sum of the cation valences in the A and B sites is 6, and the ionic radii of A ranges between 0.8 to 1.40 Å and B ranges between 0.4 to 0.9 Å. Perovskite structures are characterized by 12-fold coordinated A-site cations and six fold coordinated B-site cations. The anion lattice of perovskite materials can be described as a three-dimensional lattice of corner sharing octahedra with the A-site cations occupying the interstitial positions between the octahedra.
“Defective” perovskite structures that can generally be described by the formula(A1-xA′x)1-z(B1-yB′y)O3-δmay be achieved by substituting cations of similar radii (represented by A′ and B′) but different valence into the A and B sites. To compensate for the charge imbalance created by the cation substitution, oxygen vacancies form in the crystal structure, which is represented in the formula by the δ term. In some instances, defective perovskite structures may provide enhanced electrochemical performance.
Currently, most developmental SOFC systems operate at relatively high temperatures (i.e., about 800 to 950° C.). Operation of SOFCs at lower temperatures (i.e., about 650 to 750° C.) would minimize adverse chemical reactions between component materials, minimize adverse effects of thermal expansion mismatches between component materials, reduce cost by allowing less expensive metals to be used for interconnects and gas manifolds, and reduce the size and weight of the SOFC power generation system by lessening requirements on heat exchangers and thermal insulation.
However, it has been difficult to achieve high SOFC power densities at low temperatures in solid oxide fuel cells because of increased electrolyte resistance and inefficiency of the electrode materials. It has been demonstrated that reducing the thickness of electrolyte membranes lowers electrolyte resistance. This has been achieved in SOFCs with planar geometries by using one of the porous electrodes (typically the anode) as the bulk structural support (about one millimeter thick), depositing a dense thin film (about ten microns thick) of the electrolyte material on the porous anode substrate, and subsequently depositing the opposite electrode (cathode) as a porous film (about fifty microns thick) on the electrolyte film surface. Very high SOFC power densities have been achieved at temperatures of 750 to 800° C. with planar SOFCs produced with this type of configuration. However, even better SOFC performance and lower temperature operation are expected to be achieved by using improved cathode materials.
Two approaches have been demonstrated for improving low-temperature performance of cathodes in solid oxide fuel cells. The first approach involves addition of electrolyte material to the electrode material, which increases the volume of triple-point (air/electrode/electrolyte) regions where electrochemical reactions occur. This enhancement is most effective in LSM when ceria-based electrolytes (SDC or GDC) are added or when the particle size of the component (electrolyte and electrode) materials is reduced.
The second approach involves replacement of lanthanum strontium manganite (LSM), which conducts electricity solely via electron transport, with mixed-conducting ceramic electrode materials, i.e., materials that conduct electricity via transport of both oxygen ions and electrons. Examples of mixed-conducting electrode materials include (La,Sr)(Mn,Co)O3 (LSMC), (Pr,Sr)MnO3 (PSM), (Pr,Sr)(Mn,Co)O3 (PSMC), (La,Sr)FeO3 (LSF), (La,Sr)(Co,Fe)O3 (LSCF), and (La,Sr)CoO3 (LSC), among others. Of these materials, the LSF compositions, and particularly the cobalt-containing LSF compositions, demonstrate the lower interfacial resistance values, by virtue of intrinsic oxygen vacancy formation at the operating temperature of the cells. As the temperature of the cathode material increases, oxygen vacancies form as Fe3+ ions and Co3+ ions change valence to 2+, functioning as an in-situ dopant. The Co-containing compositions demonstrate the lowest interfacial resistance, but have high coefficients of thermal expansion (typically 14-20 ppm/° C.) that limit their compatibility with the lower-expansion electrolytes. (YSZ and LSGM typically have thermal expansion coefficients near 10 ppm/° C., while ceria electrolytes typically have a thermal expansion coefficient near 13 ppm/° C.) Low levels of cobalt doping in LSF provide an active electrolyte with limited compatibility with YSZ, but chemical interaction is still a problem at higher operating temperatures.
Accordingly, there is a need in the art for new ceramic electrode compositions that improve the performance of solid oxide fuel cells or other electrochemical devices, reduce the operating temperature of solid oxide fuel cells, and/or allow efficient operation of solid oxide fuel cells with internal reforming of hydrocarbon fuels. Such compositions also must exhibit chemical and mechanical compatibility with electrolytes during cell operation. In addition to identifying appropriate compositions, processes are required to economically produce and deposit such cathode on solid oxide fuel cells, ceramic electrochemical gas separation systems, gas sensors, and ceramic membrane reactors.
It is an object of the invention to provide new ceramic electrode compositions with high electrical conductivity and low interfacial resistance with SOFC electrolytes at operating temperatures below 800° C. It also is an object of the invention to provide new ceramic electrode composition that provide superior performance in other electrochemical devices.
It is another object of the invention to provide methods for economically producing and depositing such compositions for use as electrodes of solid oxide fuel cells, ceramic oxygen generation systems, electrochemical gas separation systems, gas sensors, and ceramic membrane reactors.
These and other objects of the present invention will be apparent from the specification that follows, the appended claims, and the drawings.